You may be interested in taking a look at SubEthaEdit. It's a Mac OS X-only application, but it's an interesting design. Basically, it uses OS X's zeroconf rendezvous communication to allow concurrent editing of documents over a network. You can see the changes that others are making and continue to make changes yourself.
in reply to (OT) Collaboration in Context
One of the cited uses is pair programming. Instead of stuffing two people in front of one computer, you can give each of them their own computer and sit them next to each other. I've never used it, but it looks interesting enough.